Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous compounds, which may include nitrous oxides (NOx), and solid particulate matter, which may include unburned carbon particulates called soot.
Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of gaseous compounds emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions is exhaust gas recirculation (EGR). EGR systems recirculate the exhaust gas byproducts into the intake air supply of the internal combustion engine. The exhaust gas directed to the engine cylinder reduces the concentration of oxygen within the cylinder and increases the specific heat of the air/fuel mixture, thereby lowering the maximum combustion temperature within the cylinder. The lowered maximum combustion temperature and reduced oxygen concentration can slow the chemical reaction of the combustion process and decrease the formation of NOx.
In many EGR applications, the exhaust gas is passed through a particulate filter and catalyst containing precious metals. The particulate filter may capture a portion of the solid particulate matter carried by the exhaust. After a period of use, the particulate filter may become saturated and may require cleaning through a regeneration process wherein the particulate matter is purged from the filter. In addition, the catalyst may oxidize a portion of the unburned carbon particulates contained within the exhaust gas and may convert sulfur present in the exhaust to sulfate (SO3).
As shown in U.S. Pat. No. 6,427,436 (the '436 patent), a filter system can be used to remove particulate matter from a flow of engine exhaust gas before a portion of the gas is fed back to an intake air stream of the engine. Specifically, the '436 patent discloses an engine exhaust filter containing a catalyst and a filter element. A portion of the filtered exhaust is extracted downstream of the filter and is directed to an intake of the engine through a recirculation loop.
Although the filter system of the '436 patent may protect the engine from harmful particulate matter, the catalyst may convert sulfur present in the exhaust gas to sulfate. As mentioned above, the formation of sulfate may cause particulate emissions to exceed regulated levels.
The disclosed exhaust treatment system is directed to overcoming one or more of the problems set forth above.